1. Field of the Invention
This invention relates generally to a capacitor microphone, and is directed more particularly to a bi-directional capacitor microphone having a pair of back-to-back transducer sections.
2. Description of the Prior Art
In general, a conventional bi-directional capacitor microphone is formed of a pair of conductive back plates and a conductive diaphragm disposed between the back plates with predetermined air gaps separating it from the back plates. In particular a first transducer section is formed of a first perforated back plate and a diaphragm spaced a predetermined amount from the first back plate, and a second transducer section is formed of a second perforated back plate and spaced another predetermined amount from the diaphragm. In such a microphone, the first and second back plates are positioned either to the left and right or above and below the diaphragm, and are electrically connected to each other. An electret layer is coated on the surfaces of both back plates that face the diaphragm. Thus, an electric charge is applied to the back plates so that vibrations of the diaphragm will induce an electric signal between the back plates and the diaphragm.
The conventional microphone described above can be provided with a bi-directional characteristic by selecting the electret of the first transducer section and the electret of the second transducer section to have opposite polarities.
Although the above microphone has a bi-directional characteristic, its frequency characteristic for low frequencies is rather poor. The reason for this is that the diaphragm is common to both the transducer sections, and the lower frequency sound waves of arrive virtually simultaneously at both surfaces of the diagram with almost no phase difference imparted to the two surfaces of the diaphragm, as a result, the diaphragm is not vibrated.
Another drawback of the above microphone is that unless the distance from the diaphragm to each back plate, that is, the width of each of the air gaps, is selected equal to the other, the characteristics of the respective transducer sections are not matched. This matching of characteristics is very difficult work in the microphones according to the prior art.